Large, dark triangular stealth aircraft with no lighting traveling south at 1500 ft just after dark.

Just after dark at approximately 7:05pm this evening, observed low flying, dark triangular aircraft directly overhead and rapidly flying due south in a straight line. Aircraft was very dimly, even faintly lit around its rounded edges with no typical aircraft lights or strobe. Observed by two adult men (professionals ages 50 and 57) for approximately 5 or 6 seconds. Though the craft seemed no higher than 1000 - 1500 feet, there was no observable sound. Due to the craft’s speed, it traveled quickly out of sight within a few seconds. Given the high use airspace and proximity to SFO, other commercial aircraft were observed much higher on this clear night with the typical faint jet engine drone. Our thinking is that this craft could possibly be some sort of stealth military aircraft that may have been flying this evening with no lights.